How long will your batteries last behind your Axpert inverter?

The formula

autonomy = capacity × health × (charge − cut-off) × efficiency ÷ (consumption + inverter’s own consumption)

Example: one 5.12 kWh battery in good health, from 90 % down to a 10 % cut-off, behind an inverter at 92 % efficiency that draws 40 W for itself, with an average consumption of 500 W: 5.12 × 1 × 0.8 × 0.92 ÷ 0.54 ≈ 7 hours.

Why real runtime is often shorter

• One weak cell ends the discharge. The BMS stops the battery when its lowest cell reaches its limit, even if the others still hold energy. A growing cell spread shortens the night.
• The displayed charge drifts. The state of charge is counted, not measured: without a full charge from time to time, it can be several points off.
• Cold reduces what you can take out. Below about 10 °C a lithium battery delivers less, and many BMS limit the current.
• Peaks cost more than the average. A kettle or a pump at 2 kW for a few minutes weighs more than its share, because of losses and current limits.

Typical night consumption in a home is 100 to 250 W; in the evening, 400 to 1,000 W. Your own figure is on the dashboard of your box after a few days.